Nitrous fumes were released at around 10:30 a.m. in a steel tube manufacturing plant during the cleaning and pumping of the retention basin for used baths in a surface treatment workshop. This operation is performed regularly by a subcontracting company during the site’s shutdown periods. The alarm was raised by the maintenance technician. The pumping operation was stopped, and the 16 employees present at the time were evacuated. Two of the employees were taken to the hospital for examination. The lorry parked in the hall was moved to an outdoor holding area. It was then gravity drained, and the firefighters sprayed it down with a protective water curtain arrangement. The tank was then rinsed, and the intervention was wrapped up at around 4:30 p.m. The operator treated the effluents recovered in the retention basin (pH at 1).

The nitrous fumes had been caused by the presence of one of the strongly basic products present in the used baths’ retention basin in greater quantities than usual. A leak on one of the bath’s pipes a few weeks earlier had created a solid deposit of this product. The amount generated was larger than the usual traces found in the retention basin.

This accident revealed other malfunctions as well. The absence of a technician dedicated to monitoring the lorry had prevented the incident from being detected rapidly. The subcontractor technicians had been assigned to other tasks. The material safety data sheets were not available on site. The fire alarm had not been used to evacuate the personnel, which would have ensured better responsiveness. The water supply connections were not compatible with the fire-fighter connections.

Following the accident, the operator required that someone be stationed next to the lorry during all material transfer operations. The lorry is now parked outdoors in an unloading area. The operator plans to address all product leakage or spillage problems as they occur.